Need help with building a computer for a friend.

Hi, my friend needs a new computer so need some help from you guys (we will build it).

Budget: 4000 DKK (using komplett.dk to buy the stuff btw)

Resolution: 1600x900

Games / Settings Desired: wow at highest possible with the budget.

Any other intensive software or special things you do (Frequent video encoding, 3D modeling, etc): nope.

Country: Denmark.

Parts that can be reused: none.

Do you need an OS? Depends, I was thinking we could download windows 8 release preview on a USB and use that, but I dont know if thats possible? if not/or if its kinda difficult then we need a windows 7. (btw if possible with the win 8 usb thing, is a 18gb usb enough?)